Electrical resonators are widely incorporated in modern electronic devices. For example, in wireless communications devices, radio frequency (RF) and microwave frequency resonators are used in filters, such as filters having electrically connected series and shunt resonators forming ladder and lattice structures. The filters may be included in a duplexer (diplexer, triplexer, quadplexer, quintplexer, notch filters, etc.) for example, connected between an antenna and a transceiver for filtering received and transmitted signals.
Various types of filters use mechanical resonators, such as bulk acoustic wave (BAW) resonators, including film bulk acoustic resonators (FBARs) and solidly mounted resonators (SMRs), or surface acoustic wave (SAW) resonators. The resonators convert electrical signals to mechanical signals or vibrations, and/or mechanical signals or vibrations to electrical signals. A BAW resonator, for example, is an acoustic device comprising a stack that generally includes a layer of piezoelectric material between two electrodes. Acoustic waves achieve resonance across the acoustic stack, with the resonant frequency of the waves being determined by the materials in the acoustic stack and the thickness of each layer (e.g., piezoelectric layer and electrode layers). One type of BAW resonator includes a piezoelectric film as the piezoelectric material, which may be referred to as an FBAR as noted above. FBARs resonate at GHz frequencies, and are thus relatively compact, having thicknesses on the order of microns and length and width dimensions of hundreds of microns.
Among other uses, acoustic resonators may be used as notch filters or band-pass filters with associated passbands providing ranges of frequencies permitted to pass through the filters. With increasing power requirements placed on devices (e.g., mobile phones), ever increasing power demands are placed on filters, and particularly the resonators of the filters. These increasing power demands can have adverse impacts on the performance and reliability of the resonators. For example, as radio frequency (RF) signals with greater electrical power are applied to known RF resonators, excessive self-heating can occur near the geometric center of the active acoustic stack, which is the farthest from the points where the active acoustic stack contacts the substrate (so-called anchor points where power is dissipated). As can be appreciated, the size of the hot spot depends on the frequency and power applied and absorbed.
The temperature gradient in the hot spot creates an active area divided into multiple resonators resonating at different frequencies, and with different acoustic properties. This temperature gradient also impacts the physical properties of the material (e.g., material stiffness), and creates acoustic discontinuities in the active acoustic stack. These acoustic discontinuities in the region of the hot spot results in further energy confinement, which is manifest in further heating at the hot spot. Ultimately, the confinement of acoustic waves and attendant concentration of thermal energy at the hot spot can cause at least bowing of the active acoustic stack in FBARs, adversely impacting the acoustic response of the resonator; and at most rupturing of the active acoustic stack and catastrophic loss of the acoustic resonator.
What is needed, therefore, is a BAW resonator that overcomes at least the shortcomings of known BAW resonators described above.